1. Field of the Invention
The present invention generally relates to objective lens assemblies and more specifically, to objective lens assemblies which provide wide field of view, low distortion, and high resolution for image intensification devices.
2. Description of Prior Art
The general purpose of any objective lens assembly is to collect light from an object scene and focus the rays into an image plane. For image intensification devices, the image plane is received directly by the intensifier photocathode. The optics are designed for color correction in the red and near infra-red spectral region, with wavelengths ranging from 0.9 microns to about 0.6 microns. The current production capabilities for intensifier tubes allow 18 mm to 25 mm image height formats, and require what is known in the prior art as a fast F# lens system on the order of F/1.2 in order to collect enough photons per degree in the field of view.
Typical third generation intensifier tubes available to the military have resolution of 40 cycles/millimeter or more over the 18 mm to 25 mm formats, and so the objective lenses must have good contrast resolution (modulation transfer function, or "MTF") out to these spatial frequencies. The objective is normally designed to focus at infinity, and closer ranges can be accommodated by slightly increasing the distance from the objective lens to the intensifier tube. The term "wide field of view (FOV)" is relative to the current state-of-the-art, and so anything wider than 40 degree circular FOV is considered "wide". Image intensifier goggles are most often designed for unity power, meaning that they introduce no magnification of the scenery in order to simulate normal human eye viewing.
The current state-of-the art for fielded military equipment is the ANVIS goggle system, which features an F/1.2 objective lens assembly with a 40 degree circular FOV, and an 18 mm image height format on the intensifier tube. The current ANVIS goggle can provide visual acuity as good as 20/40 on the Snellen ratio scale if light conditions are adequate. Resolution of this high quality is dependent first on the objective lens assembly, then the intensifier tube, and finally the eyepiece optics assembly. For certain applications, especially aircraft pilotage, a wide field of view capability is desired. There has yet been invented a objective lens assembly that increases the ANVIS goggle FOV up to 60 degrees while maintaining ANVIS resolution quality.
While the prior art has reported using objective lens assemblies none have established a basis for a specific apparatus that is dedicated to the task of resolving the particular problem at hand. What is needed in this instance is an objective lens assembly that increases the ANVIS goggle FOV up to 60 degrees while maintaining ANVIS resolution quality.